The present invention relates to a distal end optical unit for an electronic endoscope that can provide high durability and optical performance.
Recently, in most cases, an endoscope is sterilized in an autoclave (high-temperature high-pressure steam sterilizer) each time the endoscope has been used once. Therefore, the endoscope is required to have durability against sterilization by the autoclave.
To prevent steam generated by the autoclave from entering into an objective lens or an imaging surface of a solid-state image sensor, a distal end optical unit for an electronic endoscope is configured with an objective lens supporting tube in which an optical lens group is fixedly inserted and an image sensor supporting tube in which the solid-state image sensor is fixedly inserted being hermetically joined by soldering or brazing metal-plated portions thereof with melting metal material. Further, an observation port cover lens is attached to a distal end portion of the objective lens supporting tube (for example, Japanese Patent Provisional Publications No. 2000-60793 and No. 2000-115594).
Such a hermetically-sealed joint portion between the objective lens supporting tube and image sensor supporting tube provides high durability against the steam of the autoclave. Thereby, the objective lens group and solid-state image sensor, which are protected inside the hermetically-sealed tubes, are not affected by the steam of the autoclave.
However, when the objective lens supporting tube is metal-plated, peripheral light outside effective light for an endoscope observation image may be incident onto an outer circumferential portion of the objective lens and strongly reflected on an inner circumferential surface of the objective lens supporting tube, causing undesired optical flare and/or ghost.